1. Field of the Invention
The present invention relates to a method of designing an impedance transformation circuit which is applied to, for example, antenna apparatuses for multiband support cellular terminals.
2. Description of the Related Art
Multiband support portable communication terminals of recent years are required to support both frequency bands, LowBand (e.g., 824 MHz to 960 MHz) and HighBand (e.g., 1710 MHz to 2170 MHz), and an antenna apparatus therefor requires a frequency characteristic with a very wide frequency band. At the same time, miniaturization of portable communication terminals is also progressing, and thus, an antenna apparatus capable of supporting both of the LowBand and the HighBand with one antenna element is required. In such an antenna apparatus, operating modes that vary according to the frequency band are assigned to one antenna element. Normally, the antenna element is designed to support the LowBand in a fundamental mode and support the HighBand in a harmonic mode. The input impedance of the antenna element varies according to the mode (resonance point). An antenna element for a mobile phone terminal is small in size and thus has an input impedance of, for example, the order of 8Ω in the LowBand and the order of 15Ω in the HighBand.
Meanwhile, for example, an impedance transformation circuit having a configuration disclosed in JP 4900515 B1 may be used. The impedance transformation circuit is a matching circuit using a transformer circuit and can achieve impedance matching in a wide frequency band. However, when the impedance transformation circuit of JP 4900515 B1 is used to match an antenna element whose input impedance varies according to the frequency band to a feed circuit, since the transformer ratio of the transformer circuit is constant in each frequency band, if matching is achieved in one of the frequency bands, then the matching is lost in the other frequency band. That is, a multiband support antenna apparatus that uses a transformer circuit requires an impedance transformation circuit whose impedance transformation ratio varies according to the frequency band.
Meanwhile, as will be described in detail later, the smaller the inductances of a primary coil and a secondary coil forming a transformer circuit are, the more likely it is that frequency dependence occurs in the impedance transformation ratio of the transformer circuit itself.
In addition, it can be said that in a transformer with a frequency characteristic that achieves matching in a desired frequency band, the impedance on the antenna port side of the transformer is equal to the impedance of an antenna element for each frequency band.
When, in an impedance transformation circuit using a transformer circuit, the impedance on the antenna port side is matched to the impedances of an antenna in the LowBand and the HighBand, assuming a coupling coefficient that can be taken in the actual structure, a combination of an inductance L1 of a primary coil and an inductance L2 of a secondary coil which are used for coupling is limited in two ways.
The values of the inductances L1 and L2 are very small, resulting in a structure in which a desired coupling coefficient is very difficult to achieve due to factors such as those described below.
When inductors on the order of 2 nH are coupled to each other, sufficient numbers of turns of the coils cannot be secured (magnetic flux is not concentrated).
The inductance ratio of input/output units that do not contribute to the coupling of the transformer circuit increases and thus the effective value of the coupling coefficient decreases.
That is, to obtain a predetermined (large) coupling coefficient in spite of the coils having small inductances, the primary coil and the secondary coil need to be disposed to overlap each other by allowing the coils to have the same shape (a shape close to a congruence).
However, when the primary coil and the secondary coil have the same shape, it is very difficult to obtain desired (different) inductances respectively for the primary coil and the secondary coil.